Russian Ambassador Georgiy Mamedov was called in for some stern words late Tuesday from Daniel Jean, Canada's deputy minister of foreign affairs, said a government source familiar with the meeting.
Jean expressed Canada's frustration with the Russian government over the troubling situation in eastern Ukraine, said the source, who was granted anonymity because he wasn't authorized to discuss the matter publicly.
Pro-Russian gunmen seized control of government buildings in several Ukrainian cities over the weekend, demanding closer ties with Russia and more autonomy from the government in Kyiv.
"The deputy minister made clear that these attacks are clearly co-ordinated, and the Russian Federation's denial of involvement is not credible," the source said.
Jean told Mamedov that Russia must "de-escalate the situation" and that Canada — which has already imposed economic and political sanctions against Russia — is willing to take further action if nothing changes.
"The deputy also warned the ambassador that Canada is prepared to take further steps to isolate the Russian Federation, both politically and economically, if it continues down this dangerous path."
Tuesday's meeting came one day after Harper blasted pro-Russian groups for occupying buildings in eastern Ukraine cities in recent days, saying they were "Russian provocateurs sent by the Putin regime."
On Monday, Harper called the Russian actions "aggressive, militaristic and imperialistic" and "a significant threat to the peace and stability of the world."
"It's time we all recognized the depth and the seriousness of that threat," he said.